Estimating 13C enrichment in time course experiments
Gas chromatography coupled to mass spectrometry (GCMS) is being used for the dynamic analysis of metabolic pathways. Tracer experiments compare metabolite profiles under standard growth conditions to profiles at different times after switching to a nutrient source utilizing 13C-glucose. Several methods have been suggested to calculate 13C enrichment, taking into account the natural abundance of 13C and other higher-molecular-weight isotopes. If the composition of a diagnostic ion is known, a linear model can be used to partition the observed signal into proportions attributable to the unlabelled and labeled forms of the fragment. If the composition is unknown, a simple enrichment estimator is the percent change in intensity of the monoisotopic mass. A key feature of our analysis is making use of multiple diagnostic ions, affording robust enrichment estimates